Phosphorus export from pastures to waterways
Summary
The findings demonstrated that phosphorus export is dependent on a multitude of site-specific variables. There are several matters of consideration for land managers wanting to reduce the impacts of fertilizer on waterways. In particular, in rain-fed pastures, up to 25% reduction in phosphorus exports could be achieved by applying fertilizer during dry seasons, avoiding the application of the rapidly dissolving DAP to soil with a high moisture content, and thus avoiding exports of phosphorus by overland flow. For irrigated areas, the exclusive use of DAP could significantly reduce the export of phosphorus by overland flow. In addition, a 30-50% reduction in phosphorus exports could be achieved by maximising time between fertilizer application and irrigation.
Research findings
It is widely acknowledged that the application of phosphorus- based fertilizers to crops and pastures contributes to the export of phosphorus from fertilized fields to waterways. The resulting raised levels of phosphorus in waterways can have detrimental effects, including algal blooms that can compromise water resources and wetland ecology.
Given the serious impacts of phosphorus in our waterways, it is important to understand when, where and what phosphate fertilizer applications most seriously affect the export of phosphorus to waterways. Studies have shown that different fertilizers contribute various quantities of phosphorus to waterways. Similarly, increasing time between fertilizer application and irrigation or rainfall can decrease the amount of phosphorus exported.
A recent study, published in the Australian Journal of Soil Research by Nash et al (2004) examined phosphorus export from pasturelands in South Eastern Australia. The most commonly used phosphate fertilizers in these regions are DAP (diammonium phosphate), a form of fertilizer in which phosphorus dissolves more quickly, and SSP (single superphosphate), which has a slower dissolution rate.
In their recent study, Nash et al (2004) investigated the relationship between phosphorus exports from SSP and DAP due to overland flow from border irrigation as opposed to natural rainfall. DAP and SSP were applied independently to both irrigated and rain-fed pastures and phosphorus exports were measured for both types of fertilizer. Interesting, the authors found that rainfall and irrigation affected the export of phosphorus differently for the two commonly used fertilizers. While DAP contributed more dissolved phosphorus than SSP in the rain-fed scenario, under irrigation, SSP exported more phosphorus than DAP.
Why should border irrigation and natural rainfall have different affects on two commonly used fertilizers? Nash et al (2004) propose that the differing dissolution rates of the two fertilizers, in combination with the moisture content and infiltration characteristics of the soil prior to the flow event, contribute to these disparate results For the rainfall scenario, soil was already moist from previous irrigation. As such, there was limited infiltration of water into the soil and limited opportunity for soil to remove phosphorus. Nash et al (2004) suggest that the more soluble DAP quickly dissolved to export more phosphorus than SSP.
By way of contrast, in South Eastern Australia, pastures are only irrigated when there is a soil moisture deficit of at least 40mm. Thus, in the irrigation scenario, the quickly available phosphorus from DAP immediately infiltrated the soil, instead of contributing to run-off. By the time phosphorus from SSP became available, however, soil moisture content had risen and infiltration rates had dropped significantly. Phosphorus from SSP, therefore, was less likely to infiltrate the soil, and more likely to contribute to export through overland flow.
The findings of this experiment clearly demonstrate that phosphorus export is dependent on a multitude of site-specific variables. While Nash et al (2004) warn that it is premature to extrapolate legislative instruments on the basis of the site-specific results from a field study, there are several matters of consideration for land managers wanting to reduce the impacts of fertilizer on waterways. In particular, the authors maintain that, in rain-fed pastures, up to 25% reduction in phosphorus exports could be achieved by applying fertilizer during dry seasons, avoiding the application of the rapidly dissolving DAP to soil with a high moisture content, and thus avoiding exports of phosphorus by overland flow.
For irrigated areas, Nash et al (2004) suggest that the exclusive use of DAP could significantly reduce the export of phosphorus by overland flow. In addition, the authors suggest that 30-50% reduction in phosphorus exports could be achieved by maximising time between fertilizer application and irrigation.
While the results of studies like the one considered here can provide guidance for land managers, these studies also emphasise the complex interactions between phosphorus-based fertilizers, soil and water systems. It is important to consider the many site-specific variables that might influence phosphorus exports. It is clear that further research is necessary to help land managers optimise their production, while simultaneously maintaining the health of waterways in agricultural regions.
Reference
Nash D, Hannah M, Clemow L, Halliwell D, Webb B and Chapman D (2004) A field study of phosphorus mobilisation from commercial fertilisers. Australian Journal of Soil Research 42: 313-320. Read Abstract.
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